1. Field of the Invention
The present invention relates to a system for monitoring the axial load on a valve stem of a valve assembly and, more particularly, to such a system which monitors the axial load on a valve stem by monitoring the changes in elongation of the valve yoke during operation of the valve assembly.
2. Description of the Related Art
In many industries, it is important to measure the variable dynamic axial loads imposed on a work piece such as a valve stem. This is particularly true in the nuclear power industry where motor operator valves (MOV's) are extensively used. It is important that these valves are set and maintained correctly at all times since correct operation is critical to health and safety as well as to proper operation of the system in which the MOV's are used. MOV's typically include a valve, a motor operator attached to the valve through a stem and yoke means extending partially around the valve stem for connecting the operator housing and the valve housing.
The best measurement for accurately monitoring the dynamic events within an MOV is by the direct measurement of the valve stem load. It is possible to determine valve stem load from measuring axial (longitudinal) strain. The relationship between the amount of axial elongation of a material (amount of stretch/compression of a material) to the amount of load on a material is known. Therefore by measuring the axial elongation of a valve yoke, valve stem load can be determined.
It is possible to indirectly monitor the dynamic valve stem loads by measuring the strain in the valve yoke. Since valve stem load forces cause equal and opposite yoke reaction forces, the resulting yoke strain is an accurate indicator of the valve stem load.
One current device for determining the axial load on a valve stem through measurement of the strain in the valve yoke is shown in Leon U.S. Pat. No. 4,805,451 which includes a yoke strain sensor mounted directly on the yoke. As described in that patent, the yoke strain sensor is initially calibrated for the particular motor operator/valve configuration by temporarily mounting sensor means directly to the valve stem and applying a load on the valve stem and establishing the relationship between the stem strains measured directly by the valve stem sensor and the forces measured by the yoke strain sensor.
Prior art devices for determining the load on a valve stem through measurement of the valve yoke strain wherein the strain sensor is mounted directly on the yoke, such as that shown in the above described Leon patent, suffer certain disadvantages. For example, the yoke is subject to stress concentration problems in the body thereof and a sensor device mounted directly on the yoke is subject to such localized stress concentrations and therefore may not accurately measure the overall strain in the yoke. The yoke mounted sensor is also subject to local bending and/or torsional effects on the yoke, thus contributing to further inaccuracies in ultimately determining the load on the valve stem. Devices such as that shown in the Leon patent also do not compensate for temperature changes in the yoke which also can result in inaccuracies.
Accordingly, there is still a need for an improved device for accurately and continuously monitoring and measuring the axial load in a valve stem by monitoring the strain in the valve yoke. There is a further need for such a device which also can compensate for temperature changes in the yoke as measurements are being made.